The rate of nuclear fission reactions in a nuclear reactor operations are a function of the number of neutrons available to carry on the neutron triggered chain reaction. Many of the design features of nuclear reactors are based upon their impact on the neutron economy. In particular, materials for use in nuclear reactors are selected for their thermal neutron capture cross-sections, along with other properties. Low neutron capture cross-section materials are selected for reactor components, such as support structures, fuel rod claddings, moderators, etc. High neutron capture cross-section materials are selected for control rods, poison shims, etc. A "poison shim" is a high neutron capture cross-section material added in a carefully selected quantity to decrease neutron flux early in a nuclear fuel cycle, and to become transparent or neutral after neutron absorption such that late in the fuel cycle more of the fission neutrons are absorbed by the fissionable fuel.
In particular, nuclear poisons or shims are typically dissolved in the nuclear coolant fluids, such as in the water coolant loop, for reactor control in nuclear power plants, such as pressurized water reactors (PWRs) nuclear power plants. A poison shim for a nuclear reactor is selected from a chemical compound that meets the following conditions: 1. the solubility and cross-section must be such as to provide the design absorption characteristics in the coolant, and 2. the compound must be chemically and physically stable over the whole range of operating conditions of the nuclear power plant. Boron as boric acid fulfills both of these conditions having a neutron capture cross-section of 755 (0.025 eV) and a maximum required concentration of 0.32 molality, and further possesses the required physical and chemical stability over the full range of operating conditions in PWRs. Boric acid is therefore typically dissolved in the coolant loop, namely the primary water coolant loop, in PWRs. Boric acid (B(OH).sub.3) solutions are added to the PWR nuclear power plant fluid systems at the beginning of the nuclear fuel life cycle when natural fission product poisons are low. As the fission product poisons build up, the boric acid concentration is decreased. This approach allows a given load of nuclear fuel to be kept in the reactor for a longer period of time, thereby reducing maintenance costs.
The boron-10 (B.sup.10) isotope is particularly useful when dissolved in control fluids in the nuclear power plants. B.sup.10 isotope has a high thermal neutron capture cross-section and as such is responsible for nuclear reactor control due to its effectiveness in absorbing neutrons. Natural boric acid (NBA) solutions are composed of two stable isotopes of boron, namely high neutron capture cross-section boron-10 (B.sup.10 ) and low neutron capture cross-section boron-11 (B.sup.11) in an atomic ration of B.sup.10 :B.sup.11 of about 19.78:80.22. It is known, however, that the B.sup.10 isotope is responsible for nuclear reaction control due to its neutron capturing ability. The B.sup.10 isotope has a thermal neutron capture cross-section of about 3836 barns (10.sup.-24 cm.sup.2), and the B.sup.11 isotope has a thermal neutron capture cross-section of about 5 millibarns. It is desirable to separate the B.sup.10 from the B.sup.11 isotopes and provide a B.sup.10 enriched boric acid (EBA) solution in the nuclear reactor fluid systems to maximize the neutron capturing ability of these solutions, thereby allowing lower boric acid concentrations to be used which accordingly reduces corrosion levels in the primary systems.
However, B.sup.10 enriched boric acid (EBA) solutions, which contain an atomic ratio of B.sup.10 to B.sup.11 atomic ratio in excess of about 19.78:80.22 are not currently employed in reactor fluid systems since the production of B.sup.10 enriched boric acid solutions are highly expensive and uneconomical. The replacement of natural boric acid (NBA) solutions with B.sup.10 enriched boric acid (EBA) solutions in PWR nuclear power plants to effect a lower boric acid concentration in all of the reactor fluid systems, would create opportunities to achieve numerous benefits such as reduced radioactive waste volume, improved material performance, higher equipment availability, reduced maintenance, elimination of heat tracing, lower radiation levels, increased plant availability, extended fuel cycles, simplified plant operations, and potential for plant life extension.
A number of methods are known for increasing (enriching) the B.sup.10 content of boron compounds. These methods include distillation, solvent extraction, and ion exchange of boron compounds. The most significant obstacle, however, preventing the conversion of operating nuclear power plants with B.sup.10 enriched boric acid solutions is the current high cost for enriching boron. Boron-10 enriched boric acid solutions am not currently employed in the nuclear reactor fluid systems, since the enriched solutions may cost as much as $2.00-$3.00 (U.S.) per gram of 92% B.sup.10 enriched boric acid solution using presently available BF.sub.3 distillation techniques. Whereas the reactor grade natural boric acid solution may only cost $0.001 (U.S.) per gram. It has been shown in cost benefit studies that an acceptable cost of B.sup.10 enriched solutions can be at most $1.00 (U.S.) per gram of B.sup.10 enriched solution. It is clearly desirable to enrich boron containing solutions in its B.sup.10 isotope using an inexpensive process.
Researchers have investigated into the feasibility of boron isotope separation using ion exchange techniques. Kotaka, et al., "Separation of Boron Isotope by Means of Weak Base Anion Exchange Resin", Japan Chemical Society, vol. 8, p. 1482 (1973), teach a batchwise approach for boron isotope separation using ion exchange of aqueous boric acid solutions with weak base anion exchange resins. In this study, aqueous solutions of boric acid were passed through ion exchange columns containing 20-50 mesh anion exchange resin beads, depositing borate ions on the resin. The resin was then eluted with water and the B.sup.10 isotope content of the effluent fractions were found to be enriched in the B.sup.10 isotope at the end of the elution. The best separation factor achieved was 1.03 at an operating temperature of 25.degree. C., a boron loading of 50 ml of 0.101M natural boric acid solution, and an elution rate of 38 ml/hr/cm.sup.2 in a 0.8 cm.sup.2 .times.48 cm.sup.2 test column. It was also shown that higher operating temperatures and higher boron loadings tended to reduce the separation factor. The B.sup.10 enriched solution was separately collected on the trailing edge of elution.
In other work, Y. Sakuma, et al., "Boron Isotope Separation By Ion Exchange Chromatography Using Weakly Basic Anion Exchange Resin", Bull. Chem. Soc. Jpn., vol. 53, no. 7, pp.1860-1863 (1980), teach another batchwise approach to enrich a natural boric acid solution in the B.sup.10 isotope from 19.78% to 91% by feeding the feed solution through a 80-100 mesh weakly basic anion exchange resin in a 256 m long ion exchange column using water as the eluant. The natural boric acid feed concentration was 0.1 mole/dm.sup.3 and the elution rate was 20 cm.sup.3 /hr/cm.sup.2 at an operating temperature of 40.degree. C. The separation factor achieved was constant along the column and was calculated as about 1.0100.+-.0.0005 per 100 cm. The B.sup.10 enriched fraction was collected on the trailing edge of elution. However, the foregoing batchwise approaches while achieving good separation factors are uneconomical in practice since they require a plurality of batch columns connected in series to achieve the desired separation which is undesirable due in part to the high capital costs and operating maintenance costs.
Attempts have been made to reduce the cost of using enriched boric acid solutions in the fluid systems of nuclear power plants. U.S. Pat. No. 5,176,885 (Impink, Jr., et al.) teaches an in plant method integrated into existing nuclear power plant systems to effect boron-10 isotope enrichment of boric acid solutions by using ion exchange resins capable of the, thermally storing and releasing boron isotopes. Impink, Jr., by using existing boron concentration controllers and heat exchangers, effects boron-10 isotope separation in ion exchangers by repeated cold (about 50.degree. F.) deposition of dilute boric acid solutions on a strong base anion exchange resin and hot (about 140.degree. F.) elution of dilute boric acid solutions on the strong base anion exchange resin, thereby utilizing the property of boric acid to form borate anions with either one or three boron atoms contained within the molecule depending upon the solution temperature. However, after considering the operational problems likely to be encountered by operating a nuclear power plant system while enriching boron, it was decided to evaluate out of plant enrichment schemes.
It would be desirable to provide a low cost and efficient method of and apparatus for B.sup.10 isotope enrichment of boric acid solutions for use in the fluid systems of nuclear power plants.